Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Today, majority of mixed metal oxides are prepared through precipitation and co-precipitation methods followed by calcination(s) and subsequent reduction(s). Preparation of metal oxides through conventional precipitation method involves multiple high temperature steps like calcinations(s) and reduction, and these metal oxides are often activated at higher temperatures in case of their utility as bulk catalysts. Such bulk catalysts are used in production of bulk commodity chemicals such as methanol, syn-gas, hydrogen, ammonia, nitric acid, etc. Further, such bulk catalysts can also be used in synthesis of compounds of medicinal interest including drug molecules, apart from their use in synthesis of organic and inorganic molecules. A person skilled in the art would immediately realize the broad range of applications of metal oxides including in catalyst carrier(s), adsorbents, sensors, drug-delivery carriers, glass, abrasives, ceramics and coatings, in FCC and polymerization catalysis and various organic synthesis reactions.
Methanol is one of the top ten versatile chemical primarily produced from natural gas and naphtha. Apart from being itself a fuel and gasoline fuel additives, a numerous chemicals and chemical intermediates such as formaldehyde, acetic acid, MTBE/FAME, methyl acrylate, dimethyl ether (DME), chloromethane, methylamines etc. can be produced from methanol. In addition, olefins such as ethylene and propylene can be manufactured through methanol to olefins (MTO) and methanol to propylene (MTP) process, respectively. Demand for methanol is increasing rapidly. By 2016, global demand for methanol will reach at 92.3 million tons from 55.4 million tons in 2011 and as per IHS estimate, the demand for methanol will reach at 160 million tons by 2020.
Methanol is produced by converting syn-gas, obtained from steam methane reforming of natural gas, coal or biomass gasification through catalytic process. This process employs metal oxide based catalyst comprising of Cu, Zn and Al and prepared through co-precipitation method. The co-precipitation method employs metal salt of Cu, Zn and Al and Na, K hydroxides, carbonate as precipitating agent.
Dimethyl ether (DME), an important derivative of methanol, is increasingly gaining more importance as an alternative fuel because of its numerous advantages over the conventional crude oil derived fuels viz. petrol, diesel. Conventionally, DME is produced through two-step process. The first step is the catalytic conversion of syn-gas to methanol and the second step is dehydration of methanol over an acidic material such as alumina, silica-alumina or zeolite based materials which are bulk catalysts. Because of the very low equilibrium conversion of syn-gas to methanol, direct conversion of syn-gas to DME (STD) has inherent thermodynamic advantage to promote higher syn-gas conversion and thus overall economy of the process. However, embedding two reactors in a single reactor possesses significant challenges in the process. Moreover, since methanol formation and methanol dehydration have different reaction mechanisms, tuning catalytic sites for both the reactions through bi-functional and hybrid catalyst system need great and dedicated efforts.
Direct synthesis of DME from syngas requires bi-functional catalysts having two different active sites; one for methanol formation and the other for methanol dehydration. The prior art discloses that methanol formation from syn-gas is catalyzed by Cr, Cu and other active metals with preferably Cu as metal of choice. On the other hand, dimethyl ether formation from methanol is a dehydration reaction and proceeds through acid catalyzed reaction. The prior art discloses that various metal oxides such as alumina, silica, and mixed metal oxides such as silica-alumina, alumina-titania, alumina-zirconia, zeolites such as H-ZSM-5, H-beta, SAPO etc. are used for dehydration to methanol to DME. The direct conversion of syn-gas to dimethyl ether requires both metallic and dehydrating functions to be incorporated in a single catalyst entity as a hybrid catalyst. The second, third metal can be introduced in the first component of the hybrid catalyst during synthesis of the first component separately. These components may be added as geometrical spacers, structural promoter or catalyst filler with respect to active components such as alumina, zirconium, platinum, magnesium, calcium, manganese, gallium etc. as promoters.
Currently, preparation of first component of catalytic system (methanol synthesis part) for direct DME synthesis from syn-gas is carried out by co-precipitation of metal nitrate salts and a precipitating agent (usually sodium carbonate or sodium hydroxide). The catalyst obtained in the form of hydroxides or hydroxycarbonate as precursors which goes through a series of steps including (i) washing to ensure there is no residual ions; (ii) drying to make sure that all the excess water can be removed; (iii) calcination to convert the catalyst from hydroxides or hydroxycarbonate to metal oxides (in the state of the art methods of preparation, oxide of Cu is obtained as Cupric oxide, Cu (II)O); and (iv) reduction to convert the CuO in the form of Cu0 (active species for methanol synthesis).
The state-of-the-art copper based methanol synthesis catalyst has limitations and disadvantages such as large amount of solvent (i.e water) required for preparing the metal salt solution and to wash the precipitate; high temperature calcination required to convert copper/zinc/aluminium mixed hydroxide precursor to copper oxide/zinc oxide/aluminum oxide state; and need of rigorous hydrogen treatment to convert Cu(II) oxide to Cu(I) and then to Cu(0).
U.S. Pat. No. 2,400,959 discloses a method of preparation of a form of cuprous oxide which was found to be an active catalytic agent for hydrogenation or dehydrogenation of various organic compounds. Here solution of some water soluble copper salt was used, preferably cupric nitrate because of its higher solubility. Halogen salts of copper tend to produce a catalyst of lower activity because traces of the halogen salt remain in the precipitate and tend to sinter under hydrogenating conditions. Also, some other promoters were used along with glucose and NaOH. However the catalyst cuprous oxide obtained was catalytically inactive before applying activation procedures.
Further, US patent publication no. US20110105306 discloses a method for fabricating Cu—Zn—Al catalyst for producing methanol and dimethyl ether using a sol-gel method where crystal grain size, crystal type, surface structure and active sites distribution of the catalyst can be adjusted. Thus overall performance of the catalyst was improved. US2013/0211148 A1 discloses a catalyst comprising several types of combination of well known oxides for methanol/DME synthesis viz. CuO, ZnO, ZrO22 and some unpopular oxides viz. boron oxide, niobium oxide, tantalum oxide, phosphorous oxide along with the gamma alumina. The performance of the catalyst synthesized using different combinations of these oxides was found to be outstanding. However, these prior arts use rigorous calcination and reduction steps at very high temperature which makes the catalyst unstable.
There is other literature reported describing the synthesis of catalysts for methanol/DME synthesis but none of them were brought to their final form without performing calcination and directly obtaining oxide form immediately after precipitation. Thus, there is a continuous need and scope in the art of mixed metal oxide synthesis comprising better physical properties, superior stability and improved shelf life without affecting their performance in the participating reactions.
There is thus a need in the art to develop a novel method of preparation of mixed metal oxides which can eliminate the need of calcinations and rigorous reduction procedures.
The present invention satisfies the existing needs, as well as others, and generally overcomes the deficiencies found in the prior art.